Transparent conductive oxides (TCO) are electronic materials that find utilization in a large variety of optoelectronic devices including, but not limited to flat panel displays, liquid crystal displays, plasma displays, electroluminescent displays, touch panels and solar cells. These materials also used as antistatic coatings and electro-magnetic interference (EMI) shielding. TCOs are of crucial importance for a number of emerging technologies, such as organic electroluminescent devices (both displays and lighting devices), photovoltaic (PV) devices, including crystalline-Si heterojunction with intrinsic thin layer, amorphous silicon, CdTe, Culn(Ga)Se2 (CIGS), and organic photovoltaics. TCOs act as transparent conducting windows, structural templates, and diffusion barriers TCOs are also used for various optical coatings, in particular as infra-red reflecting coatings (heat mirrors) in automotive and building industries. Although the main desirable characteristics of TCO materials are common to many technologies, including high optical transmissivity across a wide range of light spectrum and low electrical resistivity, specific TCO parameters vary from one system to another. Emerging technologies require new types of transparent conductors with properties better adjusted to their needs. The number of compositions currently used as TCOs is restricted to a few primary and binary systems. This is mainly because of two factors: 1) limited bulk solubility of crystalline metal oxide phases in each other, and 2) certain technical limitations of the currently used methods. If these challenges could be overcome, it has been shown that the number of suitable transparent and conductive binary, ternary and even quaternary phases may be larger (A. J. Freeman, K. R. Poeppelmeier, T. O. Mason, R. P. H. Chang, and T. J. Marks, MRS Bulletin, 45-51, August 2000). Some of them may potentially exist in thin films only since the phase separation in this case is kinetically precluded by the film thinness.
A convenient way to make a large variety of multicomponent TCOs is low-pressure or high-pressure CVD using solid volatile organometallic precursors. However, CVD requires high substrate temperature (400-450° C.) needed for precursor decomposition. Despite the fact that this method can be applied for large area production, it is limited to thermally stable substrates (like glass and metal foils) and cannot be applied for direct TCO layer deposition onto such light absorbers as CIGS, CdTe, and organic PVs.
TCO's are commercially fabricated by magnetron sputtering. Other physical deposition techniques (electron beam evaporation, pulsed laser deposition, etc.) can also be applied. It is widely recognized that sputtering provides the best results in terms of high optical transparency and electrical conductivity of metal-oxide films, particularly ITO, ZnO, and ZnO—Al2O3. However, expensive vacuum equipment, high energy consumption (˜30 kW/m2), and batch production all contribute into the high technology cost. Additionally, PVD technologies impose certain limitations on the development of multicomponent (more than two) TCOs because of technical difficulties in controlling uniform element distribution, and thus the consistency of material properties, over time. As a result, PVD methods are not well suited for development of new TCO formulations for emerging PV systems. The progress in this area is a subject of method versatility and flexibility.
Accordingly, there is a need for solution-based technologies, which have the additional benefit of process cost reduction, as compared to PVD, being well suited for fast continuous roll-to-roll fabrication.
Solution-based preparation of different parts of PV devices (amorphous silicon layer, CIGS layer, organic PVs, CdS junction layer, TCO) has recently been the subject of intensive research. While noticeable progress has been achieved by both academia and industry in high-throughput fabrication of various PV components, suitable solution-based low-temperature TCO production remains a challenge and could be the ultimate barrier to fully solution-processed PVs. A considerable amount of effort has been devoted to developing printed TCOs. Despite certain progress in this field, none of the commercialized materials are used for PV devices mostly because of inferior conductivity-transparency properties when compared to those TCO's produced by sputtering. Known wet methods have limitations, which preclude them from reaching the goals imposed by PV technologies: sheet resistance of <7 Ω/, optical transmissivity >90% in near UV-visible-near IR regions of light spectrum, and low haze.
A number of approaches have been tried to fabricate TCOs by wet processes. Sol-gel processes are relatively slow, proceeding via a porous sediment formation and requiring high temperature for the film crystallization and densification. The chemical nature of this process does not allow for fabrication of high quality TCO. Metal-organics decomposition can be done relatively fast, but like CVD, requires high temperature for precursor degradation. This method starts from the deposition of a dilute solution or a slurry of precursor(s) in a liquid carrier, and results in the formation of porous films. Nano-solution inks for ITO fabrication have been widely commercialized for low-end applications. Their use for PV devices is still far from reality, first because of high temperature needed for sintering nano-particles, and second, because of insufficient conductivity-transparency performance. The latter is an inherent problem of sintering process, since the boundaries between the original particles cannot be completely eliminated at temperatures below 450° C. These boundaries are the reason for low carrier concentration, low charge mobility, and high haze. Oxidative spray pyrolysis is widely used for FTO (fluorinated tin oxide) and ATO (antimony-tin oxide) fabrication. It can be also used for ZnO, and possibly for other TCOs. Besides high temperature (450-550° C.) requirements, this method is not capable of high-end TCO film fabrication.
Thus, a versatile, flexible, low temperature, low cost wet process for fabrication of a wide variety of TCOs is desirable. Such a process would not only allow reduced TCO cost and fabrication of flexible devices, but also TCOs with properties that may be better adjusted for particular technology needs, resulting in improved device efficiency.